In recent years, research and development as to mobile communication systems using the spread spectrum technology based on the direct sequence spreading scheme have been extensively made. Such communication systems include a system used worldwide and known as cdmaone, and the W-CDMA system and the Wideband cdmaOne(cdma2000) system which both are aiming at the standard for IMT-2000 (see Tero Ojanperä and Ramjee Prasad, “An Overview of Air Interface Multiple Access for IMT-2000/UMTS”, IEEE Communications Magazine, September 1998. etc.). Among these, particularly for the W-CDMA, use of roll-off filters having small roll-off ratios is demanded in order to reduce the required bandwidth as small as possible relative to the chip rate. As the spread spectrum communication system for this scheme, a method combined with QPSK(quadri-phase shift keying) has been proposed.
FIG. 1 shows a block diagram of a transmitter of this spread spectrum communication system of the QPSK scheme. The transmitter of this spread spectrum communication system is comprised of a complex spreading portion 701 for performing complex spreading modulation of data Di and Dq which are the I-phase component(in-phase component) and the Q-phase component(quadrature-phase component) of the transmission signal, roll-off filters 708 and 709, a carrier modulator 716, a power amplifier 715 and an antenna 717.
Complex spreading portion 701 includes multipliers 702 to 705 for multiplying the I-phase and Q-phase data by complex number sequences W0 and W1 and adders 706 and 707 each performing addition of the I-phase and Q-phase data modulated by W0 and W1.
Carrier modulator 716 includes a sinusoidal wave generating circuit 710, a phase-rotation circuit 713 for rotating the phase of the sinusoidal wave by π/2, a multiplier 711 for modulating the data having passed through roll-off filter 708 by the sinusoidal wave and a multiplier 712 for modulating the data having passed through roll-off filter 709 by the sinusoidal wave rotated in phase by π/2, and an adder 714 for performing addition of these pieces of modulated data.
However, since the signal wave forms having passed through the roll-off filters present large dynamic ranges, a spread spectrum communication system of this type may be demanded high linearity for a power amplifier and other circuits.
To deal with the problem of this kind, Japanese Patent Publication Hei 7 No. 312391 discloses a spread spectrum transmitter/receiver in which spectrum spreading and a π/4 shift QPSK are applied in combination. In this π/4 shift QPSK, since the positions of the signal points of sequential symbols differ in phase by π/4 from each other, the envelop will not cut the point ‘0’ when the signal transits from one signal point to another, so that the variations of the envelop is known to be small. This effect can be similarly obtained when it is applied to a spread spectrum scheme.
On the other hand, as found in ‘Global CDMA II for IMT-2000 RTT System Description’ TTA, Korea (Jun. 17, 1998), a special spreading scheme based on the combination of pseudo-random sequence and Walsh sequence has been also proposed. Now, this scheme is called OCQPSK(orthogonal complex QPSK) and the block diagram is shown in FIG. 2.
The transmitter for the spread spectrum communication system based on this OCQPSK scheme is comprised of multipliers 802 and 803 for modulating the I-phase and Q-phase data Di and Dq by Walsh sequences W0 and W2, respectively, a complex spreading portion 801 for performing complex spreading modulation, multipliers 810 and 811 for scrambling the data with pseudo-random sequences PN(k), roll-off filters 812 and 813, a carrier modulator 820, a power amplifier 819 and an antenna 821.
Complex spreading portion 801 includes multipliers 804 to 807 for multiplying the I-phase and Q-phase data modulated by W0 and W2 by complex number sequences W0 and W1, and adders 808 and 809 each performing addition of the I-phase and Q-phase data modulated by W0 and W1.
Carrier modulator 820 includes a sinusoidal wave generating circuit 814, a phase-rotation circuit 817 for rotating the phase of the sinusoidal wave by π/2, a multiplier 815 for modulating the data having passed through roll-off filter 812 by the sinusoidal wave and a multiplier 816 for modulating the data having passed through roll-off filter 813 by the sinusoidal wave rotated in phase by π/2, and an adder 818 for performing addition of these pieces of modulated data.
In this spread spectrum communication system, the I-phase data Di and Q-phase data Dq are modulated by Walsh sequences W0 and W2 and further processed through complex spreading portion 801 where the signals are subjected to complex spreading modulation using complex number sequences made up of W0 and W1. Further, the output signals are scrambled by a pseudo-random sequence PN(k) uniquely allotted to the user. The signals are then passed through the roll-off filters and are processed through carrier modulating portion 820, where the signals are carrier-modulated to be output to power amplifier 819. This method has the feature of limiting the phase transition of the signal to reduce the amplitude variation of the signal by performing spread modulation using complex number sequences made up of Walsh sequences.
The method disclosed by Japanese Patent Publication Hei 7 No. 312391 is able to improve the characteristic compared to the QPSK spreading modulation, however, the degree of the improvement is not so large and hence is not large enough to meet the conditions required by W-CDMA.
The OCQPSK scheme is able to provide a certain degree of improvement, however, this is not good enough. That is, the reason why the OCQPSK cannot produce high enough improvement is that the Q-phase data Dq is spread modulated by codes represented by W2 or that the signal is varied in phase by units of 90 degrees every two chips when the signal is input to complex spreading portion 801 shown in FIG. 2. Since the restriction on phase transition is effective only when the phase of the symbol does not vary, the effect of restriction on phase transition is limited to one of two transitions, thus sharp improvement of the characteristics cannot be obtained.
It is therefore an object of the present invention to provide a spread spectrum communication system in which the requirement on the linearity for the amplifier etc. can be alleviated by inhibiting the amplitude variations of the signal more tightly than the above schemes, thus allowing for use of a compact, low-cost and energy saving transmitter.